The CAPTIF test track is divided into separate pavement test sections which will have a variety of sensors buried within them. Each section is serviced by an instrumentation cabinet. Inside each cabinet is a rack of data acquisition hardware and terminal panels that the sensors are connect to. Our current hardware is a modular system produced by National Instruments Ltd.(external link) It consists of a NI cDaq-9188 chassis populated with the signal conditioning modules required by the various sensors. Each chassis is connected by Ethernet to a controller PC in the CAPTIF control room. The controller PC runs programs coded in NI Labview(external link) software to record data from the sensors.
A fundamental measurement that has been recorded at CAPTIF is the strain that occurs in pavement materials. Both elastic strains caused by transient moving vehicles and permanent strains caused by repeated loads over time can be measured. The sensors used for these measurements are inductive coil sensors. The coils are free floating and cause much less disturbance to the materials than other types of strain sensor. The coil pairs work in both the vertical and horizontal plane and can be used to measure both compressive and tensile strains. The principle of operation for these sensors involves the mutual inductance coupling of two co-axial sensors embedded within the pavement layer. One of the coils is excited with an alternating current (transmitter coil). The other coil (receiver coil) outputs an induced alternating current signal due to the magnetic coupling between them. The coupling is proportional to the axial spacing between the coils. Hence, the signal output from the receiver can be used to quantify the pavement strain occurring between the coils. CAPTIF has modified and used a wide variety of Coil Strain measurement systems over the years.
At CAPTIF we measure the moisture content of the pavement as this has a considerable bearing on pavement strength. The CS625(external link) uses the principle of Time Domain Reflectometry (TDR) to measure the moisture in the material. An electrical pulse is transmitted along the wave guide. The time the pulse travels along the wave guide is related to the moisture content of the material being measured. Multiple CS625’s are connected to timing circuits in the NI hardware and then the moisture content is calculated in Labview(external link) software.
The LM35(external link) is a simple cheap semiconductor sensor that can be insulated and buried in pavements. It has a voltage output of 10mV/degC. These sensors are connected to NI hardware modules.
The Delta-T EQ2(external link) sensor is an electronic tensiometer used to measure the amount of soil suction pressure in the clay subgrades. The sensor has a Voltage output related to the suction pressure (kPa). These sensors are connected to NI hardware modules.
